What are the challenges of determining the structure of the Milky Way? Dark matter absorbs light from some stars, making them difficult to observe. All observations take place inside of the Milky Way. Interstellar gas and dust absorb light from stars, making some stars difficult to observe. The majority of stars are too faint or far away to be observed from Earth.

Assume that dark matter is uniformly distributed throughout the Milky Way, not just in the outer halo but also throughout the bulge and in the disk, where the solar system lives. How much dark matter would you expect there to be inside the solar system? Would you expect that to be easily detectable? Hint: For the radius of the Milky Way’s dark matter halo, use R=300,000 light-years; for the solar system’s radius, use 100 AU; and start by calculating the ratio of the two volumes.

Astronomers now think that there is a black hole with more than 4 milliion times the mass of our Sun at the center of our galaxy? Roughly how large would the event horizon of such a supermassive black hole be? a. the size of our moon b. about 4 light years across c. about 17 times the size of our sun d. about the size of an atom (so much mass really compresses the event horizon) e. this question can't be answered without knowing what kind of stars were swallowed by the black hole

Estimating the mass of the Milky Way a) Assuming the Sun moves in a circular orbit of radius 8 kiloparsecs around the center of the Milky Way, and that its orbital speed is 220 km/s, calculate how many years it takes the Sun to complete one orbit of the Galaxy. Remember to convert kiloparsecs to kilometers. b) Using the modified form of Kepler's third law (introduced in Lecture 13, for measuring the combined masses of binary stars), R³ m+ M = estimate the mass of the Milky Way enclosed within 8 kpc (Sun's orbit radius). The mass of the Milky Way inside p² I the Sun's orbit can be represented as a single mass (M) located at its center, and the mass of the Sun (m) can be considered infinitesimally small compared to the Milky Way's (i.e., m < M). c) Is this estimate of the Milky Way's mass an upper or lower limit? Explain your reasoning.